Pregnancy and lactation are periods of high calcium requirement. Inadequate maternal calcium intake and vitamin D insufficiency may adversely affect fetal and neonatal growth and maternal bone mass particularly in adolescent mothers. The investigators propose to evaluate the influence of calcium plus vitamin D supplementation during pregnancy on bone mass of lactating adolescent mothers with habitually low calcium and vitamin D, on fetal growth and neonate bone mass. Mothers are randomly assigned to receive daily calcium (600 mg) plus vitamin D3 (200 IU) supplement or placebo from mid gestation until parturition. Fetal growth parameters are assessed by standard ultrasound techniques at mid and late gestation. Maternal bone mass is assessed by dual energy x-ray absorptiometry (DXA) at 2 and 5 months postpartum and 6 months after weaning. Infant bone mass is assessed by DXA at 2 months postpartum.